Why are boats more efficient than airplanes?
They use less fossil fuels to move
2 answers
The mass of the ship and cargo are supported by the displacement of water (Archimedes Principle) whereas an aircraft (unless a balloon) is supported by lift which requires power from the engines. Therefore you can move huge masses with a ship that you could never lift into the air. If you want to move huge volumes and masses of cargo but do not care about speed, use a ship. ( I build ships :) This apples particularly if you need a steady stream of dense material, such as iron ore.
By the way human passengers are, about the lowest density cargo you can imagine. They want huge amounts of space for their mass and are usually in a hurry. Therefore they are not ideal ship cargo and passenger ships have huge volumes above water for limited draft and displacement. Therefore they are pretty much expensive toys whereas an iron ore ship or oil tanker is essential for its job.
2 answers